Source code of 6.1.3 - Trigonometric figures

10 CLS                 ' Clear the screen
11 CLEAR               ' Delete the contents of all variables
14 DEFDBL O
15 DEFSNG P
17 CONST cpi = 3.14592927#
20 REM Than this number further starts the program code
30 INPUT "Enter radius of circle" R
40 P1 = R ^ 2 * cpi
50 P2 = 2 * R ^ 2
60 P3 = 2 * R ^ 2 * (SIN(cpi / 3)) ^ 3
70 P4 = (P1 - P2) / 4
80 P5 = (P1 - P3) / 3
140 O1 = 2 * R * cpi
150 O2 = 4 * 2 ^ (1 / 2) * R
160 O3 = 6 * R * SIN(cpi / 3)
210 PRINT "P1=" P1
220 PRINT "P2=" P2
230 PRINT "P3=" P3
240 PRINT "P4=" P4
250 PRINT "P5=" P5
310 PRINT "O1=" O1
320 PRINT "O2=" O2
330 PRINT "O3=" O3
900 SLEEP
999 END                ' Completion of the programming code
1000 REM Than this number further start sub-programs

In line 30 requires the entry radius circle. What to do to not accept input a negative number? What would happen if instead of numbers entered a letter?

What is the difference between the display of calculated results for the perimeter and area? What command lines define these representations?

If the write line 17 as 17 CONST cpi = 3.14, disappears mark '#'. Why?